Having lived a previous life as a curtain maker, I don't see anything wrong with the price you were given, assuming the curtains were lined. Frankly, curtains are a right old pain in the jacksy to make - they're enormously difficult to cut out (correcting the grain, ensuring the fabric is flat and straight, ensuring you've calculated the drops properly - once cut there's no going back), handle during the process (you try hauling three widths of fabric to your machine for seaming and see how much fun it is) and you need a decent sized space to work in.
I agree that there is pretty much absolutely nothing that complex about making them, especially compared to clothing (until you consider having to work with half drop repeats and pattern matching across several pairs of curtains, hand pleating to pattern, goblet headings, beaded trims, all the things that make "made to order" special) but the sheer effort required, even for machine sewing them with header tape, is not to be sneered at.
I specialised in hand-sewn curtains (the only machine sewing was stitching the widths together) until it physically became too much for me, but I still make roman blinds by the hand stitching method. I spent way more time than I like to remember on my knees straightening fluid fabrics, discovering Laura Ashley's penchant for printing off grain (ask any curtain maker about LA and I guarantee they'll grimace) and learned an awful lot about how heavy big slabs of fabric are when interlined and lined!!